Roller cone bits, variously referred to as rock bits or drill bits, are used in earth drilling applications. Typically, they are used in petroleum or mining operations where the cost of drilling is significantly affected by the rate that the drill bits penetrate the various types of subterranean formations. That rate is referred to as rate of penetration (“ROP”), and is typically measured in feet per hour. There is a continual effort to optimize the design of drill bits to more rapidly drill specific formations so as to reduce these drilling costs.
Roller cone bits are characterized by having roller cones rotatably mounted on legs of a bit body. Each roller cone has an arrangement of cutting elements attached to or formed integrally with the roller cone. The most common type of roller cone drill bit is a three-cone bit, with three roller cones attached at the end of the drill bit. A prior art three-cone bit is shown in FIG. 4. The three-cone bit 40 includes a threaded connection 14 that enables the drill bit 1 to be connected to a drill string (not shown). The three-cone drill bit 40 also includes a bit body 16 having three legs 41 extending therefrom. A roller cone 20 is rotatably mounted on a journal (not shown) extending from each of the three legs 41.
When drilling smaller boreholes with smaller bits, the radial bearings in three-cone drill bits become too small to support the weight on the bit that is required to attain the desired rate of penetration. In those cases, a two-cone or a single cone drill bit is desirable. A single cone drill bit has a larger roller cone than the roller cones on a similarly sized three-cone bit. As a result, a single cone bit has bearings that are significantly larger that those on a three cone bit with the same drill diameter.
FIG. 1A shows a prior art single cone drill bit. The single cone bit 1 includes one roller cone 4 rotatably attached to a bit body 16 such that the cone's drill diameter is concentric with the axis of rotation 6 of the bit 1. The roller cone 4 has a hemispherical shape and typically drills out a bowl shaped bottom hole geometry. The drill bit 1 includes a threaded connection 14 that enables the drill bit 1 to be connected to a drill string (not shown). The male connection shown in FIG. 1A is also called a “pin” connection. A typical single cone bit is disclosed in U.S. Pat. No. 6,167,975, issued to Estes.
FIG. 1B shows a cross section of a prior art drill bit 1 drilling a bore hole 3 in an earth formation 2. The roller cone 4 is rotatably mounted on a journal 5 that is connected to the bit body 16. The work of the roller cone 4 breaks down into two general portions: a bottom contact zone 18 and a wall contact zone 17. Cutting elements 20 on the bottom contact zone 18 portion of roller cone 4 lead the cutting of the bore hole 3 by cutting at the distal end of the drill bit 1. Cutting elements 20 in the wall contact zone 17 ream the wall of the bore hole 3 to the full diameter of the drill bit 1.
Single cone drill bits sometimes experience difficulty while drilling through changes in the earth formation, such as when a “stringer” is encountered. A “stringer” refers to a relatively small portion of harder earth formation, such as a section of sedimentary rock, encountered within a relatively softer formation. A problem that is sometimes encountered with hard stringers is that the single cone drill bit will pivot based on the indentation of the lowermost inserts in the bottom contact zone. Because the roller cone is a unitary structure, the inserts in the wall contact zone are unable to continue cutting. This can cause the single cone drill bit to hang up and stall when it encounters a stringer while drilling. Excessive scraping action and limited crushing of the stringer by the inserts in the bottom contact zone of roller cone are thought to be causes of the single cone drill bit getting hung up by a stringer. Although this issue is especially prevalent in single cone drill bits, multiple roller cone drill bits (e.g. two cone and three cone drill bits) can experience similar difficulties in drilling into stringers.
In light of the difficulties in drilling stringers and other hard formations with prior art roller cone drill bits, and especially single cone drill bits, what is still needed, therefore, are improved roller cones that are suited to drill stringers and other hard formations.